During normal road operation of a vehicle, several factors combine to cause errors in the aiming of its headlamps. If the vehicle is not equipped with an automatic self-leveling suspension, uneven loading of the vehicle causes a change of the vehicle's attitude with respect to the road, which causes a permanent error in the aim of the headlamps. The change of the road slope in front of the vehicle, which occurs when the vehicle is at the bottom or the top of a hill, for example, causes a momentary aim error of the vehicle's headlamps. Here, the aim of the headlamps is too short at the bottom of the hill, and is too far at the top of the hill. When the vehicle is driving on a bumpy road, momentary errors in the aim of the headlamps occur.
Various known approaches for automatically aligning a vehicle's headlamps are based upon a sensed attitude or loading of the vehicle at its position on a road surface. A shortcoming of these approaches is that changes in the road slope are not sensed until the vehicle is positioned on the changed-slope portion of the road. Consequently, the vehicle's headlamps are not adjusted to correct for changes in the slope of the road in front of the vehicle.